Traffic radar systems are widely used in law enforcement. Since the introduction of the first moving radar system, there have been anomalies known as combining and shadowing. Combining may occur when the combined patrol car return signal and oncoming target's return signal is of greater strength than the actual patrol car return signal. Since there may only be a slight difference between a combined signal and a patrol vehicle signal pattern, it may be difficult for the radar system to discern. As a result, if a police officer is traveling in a city environment at a normal patrol speed of 25–35 miles per hour, for example, and an oncoming vehicle is close when the radar unit transmitter is turned on, it is likely that the patrol speed displayed on the radar unit will be the combined speed of the two vehicles instead of the actual patrol speed.
In another similar situation, radar units typically include a “hold” function in which the radar unit is powered on but in standby and not transmitting. When the officer wishes to measure the speed of an oncoming vehicle the hold is removed and the unit begins transmitting and searching for the strongest return signal as the patrol speed in the range of the patrol limits, such as 10–100 miles per hour. If the patrol vehicle is traveling 20 miles per hour and the target vehicle is approaching at 35 miles per hour, the strongest signal may be the combined signal of the two vehicles at 55 miles per hour.
Shadowing may occur at highway speeds when the patrol vehicle is closing on a vehicle traveling in the same direction. The patrol speed is erroneously determined as the difference between the speeds of the two vehicles.
One method of capturing and tracking the patrol vehicle signal is disclosed in U.S. Pat. No. 6,023,236 to Shelton. The '236 patent discloses a digital signal processor (DSP) traffic radar with an input from the vehicle's speedometer which is used to narrow a search window to +/− five miles per hour around the speedometer input speed. As a result, the patrol vehicle signal return is searched for in this narrow window eliminating the problems associated with combining and shadowing. One limitation of this and other systems with a speedometer input to the radar unit is that it may not be practical for a police department to easily access the speedometer wiring and make the necessary connection.
Another method that has been used to reduce patrol speed capture anomalies and does not require a hardwired interface to the vehicle is disclosed in U.S. Pat. No. 5,565,871. The patent describes a process for allowing the rejection of an incorrect patrol speed displayed. The operator can manually reject a captured patrol speed if the operator determines that it is incorrect. However, this system does not reduce initial patrol search errors.